Experimental studies are conducted on concurrent exposure of appropriate biological targets to numerous carcinogens of different classes, each administered at doses which would be expected to produce low or undetectable effects in the chosen system. Mechanisms of action are studied to determine the most effective conditions of synergism. The biological models selected for these studies include in vitro assays for mutagenesis and neoplastic cell transformation and whole animal models. Analysis of dose-response relationships in carcinogenesis is conducted on data collected in a comprehensive literature survey, and on dose-response results obtained experimentally in the selected biological systems. In mutagenesis tests, mutual inhibition was found for polycyclic aromatic hydrocarbons, additive effects among aromatic amines and mutual synergism for benzo[a]pyrene, benzidine, aflatoxin B-1 and safrole.